The present invention relates to an exhaust purification apparatus for internal combustion engines. More particularly, the present invention pertains to an exhaust purification apparatus that has a nitrogen oxide storage-reduction catalyst in an exhaust passage.
In a typical engine, fuel is injected into an intake port from a fuel injector to charge a homogeneous mixture of fuel and air to the associated combustion chamber. An intake passage is opened and closed by a throttle valve, which is operated by manipulating an acceleration pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of homogeneously mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
However, when performing homogeneous charge combustion, vacuum is produced by the throttling action of the throttle valve. The throttling increases energy loss due to pumping, which results when the mixture is drawn into the combustion chamber from the intake port. To attempt to solve this problem, stratified charge combustion has been proposed. In stratified charge combustion, the throttle valve is opened wide and fuel is supplied directly into each combustion chamber when the engine load is small. This provides a mixture having a relatively low air-fuel ratio in the vicinity of the ignition plug. As a result, the mixture is easier to ignite. Stratified charge combustion improves the fuel economy of the engine and decreases the energy loss of pumping.
As in stratified charge combustion, when an engine operates under a lean air-fuel ratio, a NOx storage-reduction catalyst apparatus is employed to purify nitrogen oxides, which are likely to be produced in the lean air-fuel ratio range. The main component of the catalyst apparatus is, for example, zeolite. It is presumed that zeolite temporarily adsorbs hydrocarbons in the exhaust, which reduces NOx in the exhaust. For example, Japanese Unexamined Patent Publication No 6-193487 discloses an apparatus having a NOx storage-reduction catalyst. The apparatus performs so-called rich spike control to reduce NOx. When the engine continues to be operated with a lean air-fuel ratio, NOx adsorbed on the catalyst reaches saturation, and the extra NOx is emitted in the exhaust gas. For this reason, the air-fuel ratio is controlled to be rich temporarily. The control increases the amount of HC in the exhaust gas and NOx is reduced to nitrogen gas (N.sub.2).
Also, in the apparatus of the Japanese publication, a richness ratio is set to restrain the fluctuation of engine torque under rich spike control. The fuel injection amount is calculated to equalize engine torques before and after the rich spike control. That is, the engine torque under a lean air-fuel ratio, before the rich spike control starts, and the engine torque under a rich air-fuel ratio, when the rich spike control starts, are equalized.
However, in the apparatus of the Japanese publication, the following problems occur. When rich spike control is performed during unstable engine operation (for example, (1) when stratified charge combustion is changed to homogeneous charge combustion or (2) when air intake varies substantially), engine output fluctuation occurs and misfire can occur.
First, case (1) is described. Generally, when rich spike control is performed under homogeneous charge combustion, the fuel injection amount is increased and ignition timing is delayed. However, during stratified charge combustion, there is high air intake, and fuel must be substantially increased to make the air-fuel ratio rich. During stratified combustion, since the fuel density around the spark plug becomes too high because of the fuel increase, the same control method used with homogeneous combustion cannot be employed. When performing rich spike control, the fuel injection amount is increased and the openings of the throttle valve, swirl control valve (SCV) and exhaust gas recirculation (EGR) valve are narrowed, and pumping loss is increased. This limits the increase of engine torque. Rich spike control during stratified combustion is designed to make the combustion conditions similar to those of homogeneous charge combustion. Alternatively, the combustion control may be changed to homogeneous charge combustion. In other words, rich spike control must be performed during stratified charge combustion.
Accordingly, when the combustion control is being changed between stratified charge combustion and homogeneous charge combustion, the air intake amount is changing drastically. Under this state, rich spike control tend to cause power output fluctuation and misfires.
When the air intake amount simply changes (as in case (2)), the actual opening degree of the various valves (throttle valve, SCV, EGR valve) is likely to be different from the required opening degree. In that case, air intake conditions (air-fuel ratio, swirl strength, EGR amount) tend to differ significantly from the required conditions. When rich spike control is performed under these circumstances, power output fluctuation and misfires tend to occur.